1. Field of the Invention
The present invention relates to a photothermographic material and an image forming method using the same.
2. Description of the Related Art
In recent years, decreasing the amount of processing liquid waste in the field of films for medical imaging has been desired from the viewpoints of protecting the environment and economy of space. Technology is therefore required for photosensitive thermal developing image recording materials which can be imagewise exposed effectively by laser image setters or laser imagers, and thermally developed to obtain clear black-toned images of high resolution and sharpness, for use in medical diagnostic applications. An image forming system using photosensitive thermal developing image recording materials does not require liquid processing chemicals and can therefore be supplied to customers as a simpler and environmentally friendly system.
While similar requirements also exist in the field of general image forming materials, images for medical imaging in particular require high image quality excellent in sharpness and granularity because fine depiction is required, and further require blue-black image tone from the viewpoint of easy diagnosis. Various kinds of hard copy systems utilizing dyes or pigments, such as ink jet printers and electrophotographic systems, have been marketed as general image forming systems, but they are not satisfactory as output systems for medical images.
Photothermographic materials utilizing organic silver salts are described in many documents. Photothermographic materials generally have an image forming layer including catalytically active amounts of a photocatalyst (for example, silver halide), a reducing agent, a reducible silver salt (for example, an organic silver salt), and if necessary, a toner for controlling the color tone of developed silver images, dispersed in a binder. Photothermographic materials form black silver images by being heated to a high temperature (for example, 80° C. or higher) after imagewise exposure to cause an oxidation-reduction reaction between a silver halide or a reducible silver salt (functioning as an oxidizing agent) and a reducing agent. The oxidation-reduction reaction is accelerated by the catalytic action of a latent image on the silver halide generated by exposure. As a result, a black silver image is formed on the exposed region. The Fuji Medical Dry Imager FM-DPL is an example of a medical image forming system that has been made commercially available.
Methods of manufacturing such photothermographic material include a method of manufacture by a solvent coating, and a method of coating an aqueous coating solution using an aqueous dispersion of fine polymer particles or an aqueous solution of a water soluble polymer as a main binder followed by drying. Since the latter method does not require a process of solvent recovery or the like, a production facility therefor is simple, environmental burden is small, and the method is advantageous for mass production.
However, in the method of manufacturing the photothermographic material by an aqueous coating system, since the coating solution for the image forming layer contains many components required for image formation, there is a significant problem with regard to uniformly coating and drying the same. Particularly, in a case of coating a solution at a high speed and rapidly drying the same to prepare a photothermographic material in order to enhance productivity, there are various problems such as increase of haze due to partial lack of balance among the components in the coated layer and occurrence of unevenness in the coated surface state due to fluctuation of drying wind.
In U.S. Pat. Nos. 6,630,291 and 6,713,241, use of a hydrophilic binder such as gelatin as a binder is described; however, there are problems in that it is difficult to obtain a high image density and image color tone is poor due to a large amount of fogging.
In the photothermographic material, it is necessary that chemical components necessary for forming an image are contained in the film in advance. For this reason, these chemical components exert influences on storage stability of the photothermographic material up until it is used.
Further, even after an image has been formed by subjecting the photothermographic material to thermal development, since these chemical components remain in the film as unreacted components or reaction products, these chemical components exert influences on transparency of the film and the image color tone and, moreover, exert significant influences on the storage stability of the image. Therefore, it is desirable that the number of types and amounts of these chemical components are small, and it is further desirable that, although the chemical components have a high activity in an image forming reaction at the time of thermal development, they are inactive in storage; however, such requirements as described above have not sufficiently been satisfied so far, and improvement is required.